Pelvic organ prolapse (POP) is the descent of the apex of the vagina, including the cervix (or vaginal vault after hysterectomy), anterior vaginal wall, and/or posterior vaginal wall. As prolapse progresses, pelvic organs may become displaced and even protrude outside the vaginal canal. POP is a highly prevalent condition affecting at least 50% of women in the US during their lifetimes. In fact, some loss of utero-vaginal support occurs in most adult women. POP is the leading indication for hysterectomy in postmenopausal women and accounts for 15-18% of procedures in all age groups [Kesharvarz H, Hillis S D, Kieke B A, Marchbanks P A. Hysterectomy surveillance—United States 1994-1999. MMWR Surveill Summ 2002; 51 (SS05):1-8]. Overall, 1 in 10 women will undergo surgery to treat pelvic floor support conditions in the course of their lifetime. This number is projected to significantly increase with the anticipated growth of the aging population in the United States. Beyond the physical impact of POP, women with progressing pelvic organ prolapse score poorer on both general and condition-specific quality-of-life scales [Jelovsek J E, Barber M D. Women seeking treatment for advanced pelvic organ prolapse have decreased body image and quality of life. Am J Obstet Gynecol 2006; 194: 1455-1461]. In addition, about one third of sexually active women with POP report that their condition interferes with sexual function [Barber M D, Visco A G, Wyman, et al. Sexual function in women with urinary incontinence and pelvic organ prolapse. Obstet Gynecol 2002; 99:281-289].
Women with symptomatic POP who fail or decline conservative management, including pessary use and physical therapy treatment, are candidates for reconstructive surgery. The overall goal for prolapse surgery is to give the most functional repair, while preventing recurrence of the condition and minimizing complications incurred by these repairs. Recurrence is one of the barriers in surgical correction most frustrating to both the surgeon and patient. Failure rates as high as 20-40% have been cited after surgical repair, with over 50% occurring within the first three years [Clemons J L, Myers D L, Aguilar V C, Arya L A. Vaginal paravaginal repair with an AlloDerm graft. Am J Obstet Gynecol 2003; 189(6):1612-1618]. Since many patients with POP have inherently deficient or defective connective tissue, to minimize recurrence of POP many reconstructive surgeons have turned to the use of adjuvant materials for vaginal support. Such materials may include synthetic, allogenic, xenogenic or autologous grafts [Bako A, Dhar R. Review of synthetic mesh-related complications in pelvic floor reconstructive surgery. Int Urogynecol J Pelvic Floor Dysfunct 2009; 20(1):103-111]. Currently, at least 10 synthetic materials are available for vaginal use [Sung V W, Rogers R G, Schaffer J I, et al. Graft Use in Transvaginal Pelvic Organ Prolapse Repair: A Systematic Review. Obstet Gynecol 2008; 112(5):1131-1142]. Unfortunately, none of the currently available graft materials is ideal for restoration of both optimal support and functionality of the vaginal walls.
When determining the etiology of POP and delineating risk factors for POP, parity has the strongest association with risk of requiring surgery for POP. Pregnancy and childbirth have a tremendous impact on women's vaginal connective tissue, nerves and muscles support within the pelvis due to prolonged pressure, straining and distention forces that are placed on the pelvic tissues. Traumatic changes of the pelvic floor are encountered during childbirth including avulsion of muscle from the supporting bony structure of the pelvis, damage to vaginal support ligaments and muscle atrophy after pelvic nerve damage. Pelvic floor dysfunction, in the form of pelvic floor prolapse (including cystocele, rectocele, enterocele and uterine prolapse) and urinary and fecal incontinence are considered inevitable sequelae for some women who experience injuries during childbirth. Compared with nulliparous women, women with one child were 4 times more likely (and those with two children were 8.4 times more likely) to develop pelvic organ prolapse requiring hospital admission and surgical intervention. With the burgeoning elderly population, the latent injuries caused in childbirth will affect more and more women later in life. Although surgery for pelvic organ prolapse is effective in restoring anatomy, functional outcomes have not been as satisfactory and there are many questions regarding underlying biomechanical properties of the pelvis that are currently poorly defined to guide optimal repair.
Pelvic floor organs and support structures are elements of a biomechanical system providing critically important set of physiological processes. Despite the obvious fact that POP and childbirth damages are caused by structural failures, only recently researchers have begun to conduct a biomechanical analysis of the mechanisms of normal pelvic organ support and failure.
A critical review of published data on the urogynecologic aspects of female sexual dysfunction demonstrates a lack of standardized instruments for assessing biomechanical conditions of the pelvic floor. There is a need in 3-D imaging of vagina and its surrounding structures and reproducible measurements of vaginal tissue elasticity in-vivo because the tissue elasticity, as a capability to hold load and reversely undergoing to elongation, is the primary mechanical characteristic.
The high incidence of POP, childbirth damages and the rate of reconstructive surgery dictate the need for new effective methods for assessment of pelvic organ conditions after reconstructive surgery or other interventional procedures in women. Elasticity imaging of the vagina after reconstructive surgery may allow to quantitatively characterize the effectiveness of the surgical approach and behavior of materials used for vaginal support in-vivo.
Elasticity Imaging and Assessment of Soft Human Tissues
In the last decade, a new modality for tissue characterization has emerged termed Elasticity imaging or Elastography. Elasticity imaging allows visualization and assessment of mechanical properties of soft tissue. Mechanical properties of tissues, i.e. elastic modulus and viscosity, are highly sensitive to tissue structural changes accompanying various physiological and pathological processes. A change in Young's modulus of tissue during the development of pathological processes could reach hundreds and even thousands of percent (A. P. Sarvazyan. “Elastic properties of soft tissue”, In: Handbook of Elastic Properties of Solids, Liquids and Gases, Volume III, Chapter 5, eds. Levy, Bass and Stern, Academic Press, 2001, pp. 107-127.). Elasticity imaging is based on generating a stress in the tissue using various static or dynamic means and then measuring resulting strain (displacements in volume) with the use of ultrasound or magnetic resonance imaging. Tactile imaging yields a tissue elasticity map similar to other elastographic techniques. At the same time, tactile imaging, unlike strain imaging, uses stress or pressure data on the surface of tissue under applied load. It mimics manual palpation, because a tactile imaging probe with a pressure sensor array mounted on its face acts similarly to a human finger during a clinical examination by compressing soft tissue with the probe and detecting resulting changes as a surface pressure pattern.